There has been developed a power semiconductor device using a wide band gap semiconductor made of silicon carbide (SiC), gallium nitride (GaN), diamond (C) or the like. Such a power semiconductor device, although operated at a high semiconductor joint temperature (Tj), has an on-resistance lower than that of a conventional power semiconductor device using silicon (Si) or gallium arsenic (GaAs), and thus is capable of fast switching. Therefore, reduction in chip size of the semiconductor device as well as reduction in size of a passive component and a cooler included in a system can be achieved. Thus, it is expected that a small, light and inexpensive power electronics system can be realized.
As a matter of course, high heat resistance is demanded for a joint portion, i.e., die attachment between a semiconductor element chip and a metal substrate, which are included in such a power semiconductor device operated at such a high semiconductor joint temperature (Tj). From the viewpoint of electric conductivity, thermal conductivity and price, a plate material mainly made of copper (Cu) is generally used as the metal substrate. Such a plate material is mostly used in a state of being attached to a ceramic insulating substrate. Hereinafter, substrates including the one having the plate material attached to the ceramic insulating substrate are simply referred to as a “Cu substrate”.
Meanwhile, high melting point Au-based solder is widely used as a joint material (see Non Patent Literatures 1 to 3). The Au-based solder described here includes eutectic AuGe solder (melting point of 356° C.), eutectic AuSi solder (melting point of 363° C.), eutectic AuSn solder (melting point of 280° C.), and the like.